Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS), developed by the 3rd Generation Partnership Project (3GPP™) (www.3gpp.org). The 3rd generation of wireless communications has generally been developed to support macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with wireless communication units within a relatively large geographical coverage area. Typically, wireless communication units, or User Equipment (UEs) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network. Each macro-cellular RNS further comprises a controller, in a form of a Radio Network Controller (RNC), operably coupled to the one or more Node Bs, via a so-called Iub interface.
Lower power (and therefore smaller coverage area) femto cells (or pico-cells) are a recent development within the field of wireless cellular communication systems. Femto cells or pico-cells (with the term femto cells being used hereafter to encompass pico-cells or similar) are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APs)). These femto cells are intended to be able to be piggy-backed onto the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment.
Typical applications for such femto APs include, by way of example, residential and commercial (e.g. office) locations, communication ‘hotspots’, etc., whereby an AP can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, femto cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion at the macro-cell level may be problematic.
Under certain conditions, for example when a cell becomes fully loaded with on-going communications that it is able to support (i.e. when the number of active connections with UEs for that cell reaches a maximum supported number), it may be necessary or desirable to redirect UEs attempting to connect to that cell to an alternative cell. Within a typical UMTS™ network, UEs attempt to connect to a cell by initiating a Radio Resource Control (RRC) Connection procedure. Specifically, a UE sends an RRC Connection Request message to the RNS supporting the cell to which it is trying to connect. If the UE is to be allowed to establish a connection, the RNS transmits an RRC Connection Setup message back to the UE comprising connection settings, etc. for the UE. Upon receipt of the RRC Connection Setup message, the UE applies the connection settings specified in the RRC Connection Setup message, and sends an RRC Connection Complete message back to the RNS to confirm that the settings have been applied. However, if, say, the cell through which the UE is attempting to connect is fully loaded and a connection cannot be established with the UE, it may be desirable/necessary to redirect UEs from which RRC Connection Request messages are received to alternative cells.
In the 3GPP™ UMTS™ technical specifications, and in particular 3GPP TS 25.331 of the UMTS technical specifications, an RNS is able to specify within an RRC Connection Reject message (i.e. a message rejecting the RRC Connection Request from a UE) an alternative frequency, within a “frequency info” Information Element (IE) of the RRC Connection Reject message, or an alternative Radio Access Technology (RAT), within an “inter-RAT info” IE of the RRC Connection Reject message, that the requesting UE should switch to. In this manner, for network configurations where there are known to be suitable alternative cells that are ‘Inter-Frequency’ (i.e. cells arranged to use different carrier frequencies to that of the current cell) or ‘Inter-RAT’ (i.e. cells arranged to use different radio access technologies to that of the current cell), the RNS is able to redirect a UE that is attempting to connect thereto, to an alternative carrier frequency and/or an alternative radio access technology, and thereby to an alternative cell. For example, the RNS may know of alternate cells from OAM configuration, from measurements made by receivers connected to the RNS or from measurements received from the UE.
However, a problem with this approach is that it is only applicable where there are known to be (at least to the RNS that is supporting the cell to which the UE is attempting to connect) suitable alternative Inter-Frequency or Inter-RAT cells. For example, for network configurations where there are only alternative Intra-Frequency cells (i.e. cells arranged to use the same carrier frequency and same RAT as the current cell), the RRC Connection Reject message defined in the 3GPP technical specifications does not provide a mechanism for redirecting a UE to such an alternative Intra-Frequency cell. Accordingly, upon receipt of an RRC Connection Reject message, a UE will simply re-attempt to establish a connection with the same cell, transmitting another RRC Connection Request message to the RNS. As a result, the UE will fail to establish a connection until, say, the cell to which it is attempting to connect becomes less busy and a connection therewith becomes possible, potentially resulting in a poor user experience, or another non-fully loaded cell becomes more favourable (for example due to the UE moving from one cell coverage area to another).
An alternative mechanism available within the 3GPP technical specifications for causing UEs to be redirected to alternative cells is to mark the current cell as being ‘barred’ within a System Information Block 3 (SIB3) that is broadcast within a system information message to all UEs within that cell. In this manner, UEs will not even attempt to establish a connection with the barred cell. However, a problem with this method is that the SIB3 broadcast message of a cell is read by, and valid for, all UEs in idle mode (i.e. including those UEs not involved with any active RRC connections) camped within that cell. Accordingly, upon marking the cell as being ‘barred’ within the SIB3 for that cell, all UEs in idle mode camped within that cell will simultaneously move to alternative cells, thereby potentially resulting in a large amount of unnecessary simultaneous signalling overhead for those other cells. The barred state of the cell will also prevent any new calls etc. being established on that cell, and may lead to a following period of under utilisation of the cell.
Thus, a need exists for an improved method and apparatus for redirecting wireless communication units, particularly when a communication cell is fully loaded.